1. Field of the Invention
This invention relates to a rolling mill for rolling a strip material or the like, which passes through upper and lower rolling rolls, to a predetermined thickness, and a method for operating the rolling mill.
2. Description of Related Art
In an ordinary rolling mill, upper and lower work rolls are rotatably supported inside a housing via work roll chocks, and the upper and lower work rolls are opposed to each other. Upper and lower backup rolls are also rotatably supported inside the housing via backup roll chocks, and the upper and lower backup rolls are opposed to the upper and lower work rolls, respectively. A screw down device for imposing a rolling load on the upper work roll via the upper backup roll is provided in an upper portion of the housing.
Thus, a strip is fed from an entry side of the housing, and passed between the lower work roll and the upper work roll given a predetermined load by the screw down device via the backup roll, whereby the strip is rolled to a predetermined thickness. The rolled strip is delivered from a delivery side of the housing and supplied to a subsequent step.
In the foregoing rolling mill, hysteresis during vertical control of the work rolls and backup rolls in the housing needs to be minimized in a rolling condition under a screw down force to control the thickness of a rolled plate at high accuracy. For this purpose, gaps are formed between the work roll chocks and backup roll chocks and the housing. Thus, even though deformation in an inward narrowing amount is caused to the housing under the screw down load during rolling, gaps are present between the roll chocks and the housing, so that the horizontal dynamic stiffness of the rolling mill may be low. If rolling is performed with a high rolling force and a high percentage reduction in the thickness of the strip while the horizontal dynamic stiffness of the rolling mill is low, great vibrations probably attributed to, for example, friction between the strip being rolled and the work rolls (hereinafter referred to as mill vibrations) occur in the housing or the work rolls, thereby impeding high efficiency rolling.
The applicant of the present application filed Japanese Patent Application No. 2000-187163 (Japanese Unexamined Patent Publication No. 2001-113308) as a solution to the above-described problems. The invention of this application has upper and lower work rolls as a pair and upper and lower backup rolls as a pair rotatably supported in a housing via roll chocks; a screw down device provided in an upper portion of the housing for imposing a predetermined pressure on the upper work roll; and hydraulic cylinder mechanisms provided on an entry side and a delivery side of the housing, the hydraulic cylinder mechanisms being capable of thrusting the roll chocks in a horizontal direction. According to this configuration, the hydraulic cylinder mechanisms are actuated during rolling to eliminate the gaps between the roll chocks and the housing, thereby improving the horizontal dynamic stiffness. As a result, mill vibrations are suppressed, permitting high efficiency rolling.
Truly, mill vibrations can be suppressed by eliminating the gaps between the roll chocks and the housing through actuation of the hydraulic cylinder mechanisms during rolling. Further studies and experiments conducted by the applicant, however, showed that the optimal pressing force exerted on the roll chocks by the hydraulic cylinder mechanisms varied according to the rolling condition.